Silicate-containing refractory components are widely used in aluminum casting operations for containment of molten aluminum. Examples of these include refractory liners, bricks, boards and casting mould components. The silicate adds strength, heat conductance and resistance to thermal shock. However, addition of silicate to the components also introduces some inadequacies, which have received increased attention in the past few years.
A common problem with silicate-containing refractory components is that they react with the molten aluminum or magnesium, which results in the Si02 component being reduced to Si, which then becomes dissolved in the melt, resulting in gradual deterioration of the refractory.
In refractories intended for use as bricks, castable mixtures, ramming mixes and the like it has been found that the addition of barium sulphate, carbonate or oxide to a “green” refractory mix before firing, yields a refractory that is more resistant to aluminum attack once it has been fired. Such refractories have found used in melting and holding furnaces for molten aluminum, and various troughs and similar vessels for transferring molten aluminum.
U.S. Pat. No. 4,992,395 (Dulberg et al.) discloses moldable mixture of fibres (alumina, aluminum silicate, mullite, calcium aluminum silicate, mineral wool or silicon carbide), colloidal silica, binders (organic polymers, particularly those with polar groups), 1 to 15% barium sulphate (for example in the form of the mineral baryte) which are formed into mouldable mixtures using water or water-ethylene glycol mixtures, shaped and fired at 1500° F. (815° C.) before use. The mixture showed excellent resistance to molten aluminum.
U.S. Pat. No. 4,762,811 (Vayda et al) discloses an hydraulic setting castable refractory containing aggregate (such as fused bauxite, calcined bauxite, alumina or kaolin or other alumina refractory materials), binder (such as calcium aluminate, calcium silicate, lignin or phosphates), and barium sulphate plus zinc borosilicate, that latter two providing aluminum anti-adhesion properties with barium sulphate forming the larger part of the two. The mixture can be used by mixing with water and used as conventional castable products (ramming mixes, bricks or other shapes). The combination of two components optimizes both resistance to molten aluminum and load bearing capability since reduced quantities can thereby be used.
U.S. Pat. No. 4,126,474 (Talley et al) discloses a refractory comprising a phosphate bonded plastic, ramming, mortar or castable including 0.5 to 30% barium sulphate, an aggregate (alumina-silica refractory, pyrophyllite, calcined fireclay, kaolin, bauxite, alumina, or tabular or fused alumina) and a binder (preferably a phosphate binder but also calcium aluminate, lignin or hydraulic binders). The mixture is used in unfired or fired form (but unfused). The resistance to molten aluminum penetration is said to be enhanced without loss of other properties and the mix stability in the preferred phosphate binder case is enhanced)
U.S. Pat. No. 6,008,152 (Guillo et al) discloses a refractory containing vitreaous or amorphous silica plus 0.1 to 10% barium sulphate manufactured into products preferably by slip casting and firing, for example, at over 1050° C. to make a product with superior resistance to molten aluminum.
U.S. Pat. No. 6,548,436 (Prior et al) discloses a mullite refractory formed by mixing a slurry of clay or kaolin with a water-insoluble barium or strontium compound (2 to 25%), dehydrating to create a shapeable material, forming shapes and firing at least 2650° F. (1455° C.). A range of barium or strontium compounds are suggested including carbonates, chlorides, chromates, hydroxides, sulphates, oxides but the insoluble (and non-hydrophilic) sulphate or carbonate is preferred. The firing forms a mullite from the clay that is free of cristobalite and shows superior resistance to molten aluminum. The sulphates are converted to oxides at the firing temperature.
U.S. Pat. No. 3,078,173 (Dolph) discloses a refractory material containing high concentrations of alumina (e.g. from bauxite or other alumina materials), binders (e.g. clay, lignin etc), silicates, plus 1 to 30% alkaline earth oxides or carbonates (e.g. barium or calcium) and fired, for example, at 2550° F. (1400° C.). The fired material showed improved resistance to molten aluminum.
U.S. Pat, No. 2,997,402 (McDonald et al) discloses a non-fused refractory material containing boron oxide, calcium oxide, alumina and up to 15% of other oxides including magnesium, barium, beryllium, zirconium, zinc, vanadium, chromium or molybdenum fired, for example, at 1375° C. The material contains some glassy phases and is resistant to molten aluminum.
U.S. Pat. No. 2,912,341 (Ricker) discloses a calcium aluminate bonded refractory cement (refractory aggregates including calcined fire clay, alumina or chrome ore, kyanite, olivine, fire clay and vermicullite) with 0.25 to 2.25% of an alkaline earth carbonate (e.g. barium, magnesium, strontium or calcium carbonate) fired , for example at 1700° F. (925° C.). It is suggested that the presence of the alkaline earth carbonate catalyses the formation of a ceramic bond at a lower temperature without affecting other properties.
The preceding materials require pre-mixing of the appropriate barium salt into a refractory mixture and generally firing or heating the material to be effective.
Barium sulphate or carbonate slurries (the sulphate and carbonate being almost insoluble in water) have been used for protecting surfaces from molten aluminum by forming a surface layer resistant to molten aluminum.
GB580916 (Lucas) discloses a method of protecting refractory and metal articles from attack by molten aluminum by applying a coating comprising a carbonate or a sulphate of group II elements. Barium is mentioned as one of the group II elements. It is also stated that the coating may be dried and heated, or allowed to contact molten aluminum to achieve drying and heating.
U.S. Pat. No. 6,066,289 (Eckert) mentions that one can coat a refractory trough with barium sulphate or carbonate, although no details are given.
Protective coatings have a limited protective life as they tend to spall under the thermal stresses introduced, particularly if a coated component is thermally cycled.
A further class of hydrated calcium silicate based refractory components, e.g. refractory boards, that are not fired at high temperatures before use, are widely used in handling molten aluminum since they are readily formed or machined into shapes (e.g. for casting moulds). These unfired components are well known in the art and include, for example N-17™ board sold by Pyrotek Inc which is a graphite fibre reinforced hydrated calcium silicate material.
U.S. Pat. No. 4,690,867 (Yamamoto et al) discloses a composition and manufacture of a typical un-fired hydrated calcium silicate refractory formed by combining in an aqueous slurry lime/silica mixtures with xonotlite slurry (xonotlite is a hydrated calcium silicate), wollastonite (a calcium silicate mineral) and a reinforcing fibre (e.g. carbon or alkali resistant glass fibre), and hydrothermally processing the slurry in a autoclave to form the finished material. Typically the hydrothermal process exposes the slurry to steam curing at 205° C./17 kg/cm2.
European Patent Application EP 0 763 392 (Huttner et al) similarly describes a calcium silicate refractory formed by combining in an aqueous slurry, lime, silica, wollastonite, xonotlite or tobermorite, small amounts of cellulose fibre, optionally calcium silicate fines, and carbon fibre reinforcement. The slurry is dewatered by applying a pressure of 10 to 30 bars to the slurry in a mould, then autoclaved at 7 to 14 bar. This produces a matrix of tobermorite (a hydrated calcium silicate) containing wollastonite, cellulose fibres and graphite fibres, which can then be dried in air or inert gas to remove excess water.
Refractory board of this type is used, for example, to manufacture transition plates, dip-tubes and floats, and similar components where a reasonable degree of precision in shape is required and the machinability and easy formability of the materials is an advantage.
U.S. Pat. No. 4,897,294 (Libby et al) describes the use of a composite similar to the above (containing lime, silica, wollastonite, vermiculite (a hydrated Mg—Fe—Al silicate) and organic fibre reinforcement) which is slurried and moulded under pressure to form a shape. As in previous cases, the composite was then hydrothermally treated by autoclaving at 170° C., then dried of excess water at about 110° C. The resulting material was cut to shape for used as the “hot top” in a mould for casting Al—Li alloys.
U.S. Pat. No. 4,430,121 (Shima) describes the manufacture of a material suitable for covering a crucible of molten metal with a “floating” cover, by forming a slurry of lime, silica, xonotlite, wollastonite and optionally alkali resistant glass fibre, and forming a shape by dehydration moulding, controlled to achieve a target density. The shape is cured in steam in an autoclave at a stream pressure of 6 to 20 kg/cm2, then dried in air at about 110 to 130° C.
The materials however have limited high temperature life since the hydrated calcium silicates undergo transformations that weaken manufactured parts, and as in other silica containing refractories, they will react with molten aluminum.
It is an object of the present invention to provide a simpler but still effective method of providing the protective effects of a barium (or similar salt) against molten aluminum which can be used on already formed components or more readily introduced into refractory mixes than heretofore.
It is a further object of the present invention to provide an unfired refractory board having superior thermal properties coupled with resistance to molten aluminum and magnesium alloys.